1. Field of the Invention
The present invention relates to organic electroluminescence elements and manufacturing methods thereof, and particularly relates to an organic electroluminescence element used for a display device or lighting, and a manufacturing method thereof.
2. Description of the Related Art
An organic electroluminescence element (hereinafter referred to as an organic EL element), which has a structure in which an organic layer including a luminescent layer is sandwiched between an anode and a cathode, is a device that takes out luminescence by injecting holes through the anode and electrons through the cathode and by recombining the injected holes and electrons inside the luminescent layer.
The organic EL element is suitable for cost reduction and making the element flexible, since the organic layer composing the organic EL element can be easily formed by a wet process such as a printing method.
Furthermore, the application of the organic EL elements for use such as the main display of mobile phone has already started. However, the organic EL elements are inferior to competing liquid crystal displays in terms of capability such as the half-life of the display luminance and the cost reflecting the manufacturing method, and improvement is needed.
Particularly, with regard to the manufacturing method, the organic layer of the currently-available commercial organic EL element is formed by the vacuum deposition, which does not make the best use of the features of organic EL.
On the other hand, the manufacturing method forming the organic layer by the wet process has advantages in efficiency for using material, production time, and cost for manufacturing the device. Furthermore, upon application to the display, pixels ranging a large area are painted separately by the printing method. Accordingly, problems such as uneven surfaces or a deflection in a metal mask used for patterning in the vacuum deposition do not arise.
On the other hand, when forming multiple organic layers by the wet process, there is a problem that it is difficult to form multiple layers, since a lower layer generally elutes to a solution when the solution for an upper layer is dropped. The capability of the organic EL element improves by stacking multiple organic layers which have various functions. Accordingly, this is a significant problem. For this reason, the capability of the organic EL element manufactured by the wet process is significantly lower then the organic EL element manufactured by the vacuum deposition. In order to achieve practical use of the organic EL element manufactured by the wet process, it is essential to develop a device structure suitable for the wet process and its manufacturing method.
Patent Literature 1 (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-527542) discloses a device structure of the multilayered organic EL element by the wet process and its manufacturing method. According to Patent Literature 1, the hole injection layer made of aqueous organic material is formed on a transparent electrode on a substrate. This layer does not elute on the organic layer. The hole-transporting material including cross-linker is formed using organic solvent, and the film is cross-linked and insolubilized through an optical process after forming the film. Next, as a luminescent layer, the third layer made of a luminescent organic material is formed of the organic solvent. Finally, the cathode is formed by vapor deposition method, forming the element.
The organic EL element with the above described structure disclosed in Patent Literature 1 is high in capability such as the driving voltage, the luminescence efficiency, and the product life. However, water-soluble conductive material such as the compound of Poly(3,4-ethylenedionxythiophene)) (PEDOT) and Poly(styrenesulfonate) (PSS) shown in the chemical formula below and used for the hole injection layer are generally acidic solution, and cause a problem to corrode devices such as an ink-jet nozzle.
(Chemical Formula)

In addition, this compound is not a complete solution but dispersed fine particles, and thus there is a problem of clogging the ink-jet nozzle. Furthermore, since the conductance is too high, when even at least a part of the film contacts the cathode, the leak current increases.
Omitting the hole injection layer which is the first layer and injecting holes directly to the hole transport layer which is the second layer are solutions to the problem caused by the use of the ink-jet nozzle. This procedure not only solves the problem but also is significantly advantageous to the manufacturing cost such as the manufacturing devices and production time. However, when forming the hole transport layer which is the second layer directly on an anode made of, for example, commonly-used indium tin oxide (hereinafter referred to as ITO), the holes are not fully injected, significantly reducing the luminescence efficiency and the product life. This is a problem significant in application-type hole transport material which requires mixture of the cross-linking agent and with which hole transporting property decreases.
In contrast, Patent Literature 2 (Japanese Unexamined Patent Application Publication No. 2007-288071) discloses a structure which prevents the problem caused by the use of the ink-jet nozzle and the reduction in the hole-transport property. Patent Literature 2 discloses forming a metal oxide layer made of, for example, molybdenum oxide or vanadium oxide which has large work functions and are advantageous for injecting the holes with regard to the energy level, as an inorganic hole injection layer. These are insoluble to organic solvents, and thus they do not cause a problem of elution when the organic solvent is wet-applied on the metal oxide layer.
Furthermore, for example, Patent Literature 3 (Japanese Unexamined Patent Application Publication No. 2002-222695) discloses a method for painting the organic material separately by a printing method using the wet process. According to the method in Patent Literature 3, using the water repellant insulating layer called bank determines the luminescent part of the organic EL element in an opening where the bank is not formed, and holds the organic solvent on the surface of anode at the hydrophilic opening.